In the area of numerically controlled machine tools, but also more generally in the area of automation technology, the topic of reliability plays an increasingly important role. The continuously progressing replacement of purely mechanical reliability devices such as, e.g., end switches with program-monitored reliability concepts increases the demands on the reliability of position detection and monitoring. Thus, it is demanded that a single fault (sensor failure, cable rupture, etc.) at least must not result in faulty position detection that is not recognized as such. If a faulty position detection is recognized, then at least it is possible to achieve a safe state via an appropriate reaction, e.g., via an emergency stop.
In order to achieve this, a two-channel detection of the position values using different position measuring devices is customary, the detected position values being compared to each other. This comparison is also performed in two channels. This is achieved in that two processors exchange and compare their position values respectively obtained from a position measuring device. Even if only one of the two processors detects a deviation, a safe state is established, for example, by forcing all spindles in a machine tool to a standstill.
A complete two-channel system, however, is relatively costly. For this reason, a device is described in German Published Patent Application No. 102 00 436, which makes it possible to implement reliable position monitoring on the basis of the output signals of a single position measuring device. The device makes use of the fact that in a position measuring device it is possible to form different signals representing the position value, which can be checked for consistency and which satisfy the demands on a reliable position detection. Thus, by monitoring the amplitude of periodic output signals, it is possible to ensure that the respectively detected position value is correct, or faulty signals may be recognized as such and suitable measures introduced. Data formed in different manners from the signals of the position measuring device supply position values that are derived independently of each other, which may then again be compared to each other in two channels.
It is also conventional in numerically controlled machine tools to perform lag error monitoring in the course of controlling the spindles of the machine tool. For this purpose, the setpoint input (e.g., position or speed of a spindle) is compared to the current actual value. If the deviation is too great, then the machining of the workpiece is interrupted or at least an alarm is output, since it is no longer guaranteed that the machined workpiece corresponds to the specifications. Such lag error monitoring, however, is not suitable for reliably monitoring a numerically controlled machine tool.